1. Field of the Invention
The present invention relates to ultrasonic imaging probes designed for medical applications, and, more particularly, to an improved probe wherein diagnosis and high intensity ultrasound modalities are combined in the same apparatus.
2. Related Art
Ultrasound is used in many different domains for purposes of inspection and medical diagnosis. Transducers capable of sending and receiving ultrasonic energy are commonly made from a piezoelectric material such as a ceramic, crystal or co-polymer which reacts to produce an output in response to electrical or mechanical stress. Until now, imaging modalities are often segregated from Doppler, therapy or treatment modalities because low intensity ultrasound propagating at high frequency is highly attenuated by the tissue being diagnosed and this results in low efficiency Doppler operation. Such low intensity, high frequency ultrasound produces no therapeutic or damaging (non-ionizing) effects on the tissue under test and because of this non-ionizing characteristic, ultrasound is the preferred diagnostic modality for fetus and pediatric applications.
Another factor which governs ultrasonic transducer characteristics is the energy transfer of the ultrasonic transducer device. In this regard, the energy conversion factor of the transducer device is substantially constant and depends on the type of piezoelectric material employed therein. Thus, a balancing exercise must be used by the designer in determining the best compromise between bandwidth and transducer sensitivity. Otherwise, most transducers can be designed to exhibit broad bandwidth operation but only at the expense of low sensitivity in that frequency range.
Typically, an imaging ultrasonic transducer has maximum bandwidth (obtained through the use of low quality factor piezoelectric material) in order to cover the expanded frequency range of the signal being returned through the propagation medium. Moreover, the higher the frequency of transducer, the better the image. When the transducer is used in Doppler operation, the frequency of transducer should be determined based on the velocities of the test object and the depth of the region to be explored. In general, the Doppler frequency is always lower than the associated imaging frequency so as to improve the sensitivity of the received signals.
With regard to high intensity ultrasound, if a therapeutic effect is to be produced in the sonified region, the power of the transmitted ultrasonic energy must be increased in proportion, thereby resulting in heating of the region of tissue of concern. Because conventional imaging transducers are designed with a maximized bandwidth, the application of high power electrical energy to the low quality factor-based transducers used for imaging will rapidly destroy the corresponding transducer because of excessive heating of the transducer core. As a consequence, considering the situation described above, supplying high intensity ultrasonic energy to the tissue under diagnosis dictates the use of a transducer of a particular construction, e.g., a transducer made from high quality factor piezoelectric material, as well as lowering of the frequency used, and, if necessary, cooling of the active transducer material by addition of a heat-sink or an active cooling system.
A further aspect of the application of ultrasound which was not discussed above concerns the new advanced imaging mode referred to as harmonic imaging. In this mode, the transducer must be capable of emitting ultrasound at a fundamental frequency and receiving returned echoes at two, or more, times this frequency. Further, in the case where contrast agents are injected in the blood flow, the transducer must be driven to produce a high power emission in order to collapse micro-bubbles in the contrast agents prior to receiving non-linear responses from the region of interest. It is preferable to control collapsing of the contrast agents by using another transducer operated at a lower frequency specifically tailored for this purpose.
It will be understood from the foregoing that implementing different modalities, such as imaging, Doppler or therapy operations, requires many changes between various probes. This is time consuming and, furthermore, is sometimes impossible, as a practical matter, when scanning of the image is required in guiding the operation to be carried out.
The prior art includes a number of references wherein plural functionalities are combined in the same transducer probe. In U.S. Pat. No. 4,492,120, to Lewis et al., a transducer assembly is provided which comprises separate imaging and Doppler transducers. Each transducer is independently damped according to the performance criteria of the corresponding function. The imaging transducer may be of a linear array type and the Doppler transducer is assembled on the sides of the imaging transducer. Such a construction results in a significant increase in the length of the resultant transducer device and further, the Doppler acoustic pattern is not necessarily included in the image. In general, this concept has now been abandoned and replaced by a technique wherein an array of elements are driven as Doppler transmitterxe2x80x94receivers alternately with an imaging mode of operation.
In U.S. Pat. No. 5,195,519 to Angelsen, a dual function probe is provided, similarly to the Lewis et al. patent. In one of the aspect of the Angelsen patent, the probe is comprised of a steering transducer having double emitting faces. Each face is supplied with a selected frequency so as to be compatible either with an imaging mode or with a Doppler mode. This construction is limited to single element transducers and requires a coupling bath to be operable.
U.S. Pat. No. 4,097,835, to Green, discloses a moving pair or set of focused transducer members which are movable along linear paths. Each transducer member is of a semi-circular shape so the pair taken together forms a circular surface. A first semi circular transducer can be used for B-mode imaging while the second is dedicated to Doppler functions. Because the transducers are completely separated, interference between signals produced during the imaging and Doppler modes can be avoided. However, such a configuration results in a dramatically inferior lateral resolution of the image as well as in substantially inferior Doppler spatial measurements.
U.S. Pat. No. 3,952,216, to Madison et al., discloses a multi-frequency transducer including a first transducer array operating at low frequency and a second transducer array operating at high frequency. The second transducer array is located at the front of device with the first transducer array being disposed there behind. For both transducer arrays, the arrays are formed by a plurality of single elements connected in parallel, and each single transducer element is formed by sandwiching together a plurality of piezoelectric layers. The high frequency transducer array, is used in transmitting high frequency waves in the propagating medium while the low frequency transducer array is dedicated to reception of the low frequency response obtained from the difference of the two consecutive transmitted pulses. Ultrasonic transducers of this type are well adapted for sonar (underwater) applications where the bandwidth is very narrow and sensitivity must be absolutely preserved. However, such transducers are not suitable for high resolution imaging applications and do not employ a multi-element construction.
A further multi-layer transducer construction is disclosed in U.S. Pat. No. 5,957,851, to Hossack, wherein the transducer is comprised of first and second piezoelectric layers, and the second layer is disposed on the first layer. The first and second layers are separately driven and signals from one, or the other, may be isolated each other. The combination of the two layers enables transmission of ultrasonic waves that are controlled in frequency. Echoes returned from the area of examination can be analyzed by either the first or the second piezoelectric layer or by a combination of the two lawyers. The electrical connections of the piezoelectric layers are also described in the patent. However, the transducer as described in the patent requires that the associated system be equipped with driving electronics compatible with a switching layer device, in that, otherwise, when only one of the layers is used for the reception of echoes, acoustic waves propagating through the other layer will create interference that dramatically degrades the pulse shape of the echoes.
U.S. Pat. No. 5,558,092, to Unger, discloses a method for performing a diagnostic ultrasound operation simultaneously with the application of therapeutic ultrasonic waves. A therapeutic array transducer is located at a central region of the overall array and is surrounded by the imaging transducer array. The transducer array are not necessary disposed in a linear arrangement and can be arranged in a matrix or as a combination of annular and linear array, or like confirmations. Typically, the therapeutic transducer array operates at a lower frequency than the imaging transducer array and serves as a high intensity ultrasound transmitter while the imaging transducers are used in both transmitting and receiving operations. This approach is useful in high intensity ultrasound energy applications but the image obtained is affected by the missing zone corresponding to that occupied by therapeutic transducer surface, and further, the dimensions of the array are significantly increased and thus may cause discomfort, in use, to the patient or operator.
In U.S. Pat. No. 5,769,790, to Watkins et al., an ultrasonic device is provided which comprises a combination of a therapy focused transducer and a imaging phased array transducer. In one configuration, the imaging array transducer is located at the center of the hemispherical therapy transducer, and in another configuration, the two transducers are mounted in the same plane, one next to the other. This device is capable of delivering the high acoustic energy necessary to raise by several degrees the temperature of the tissue being examined. The chief drawback of this approach is the large dimension or surface area of the resulting device. In this regard, the focal length of the therapy transducer is predetermined by the surface shape of transducer so that the transducer device must be moved according to the location of the area of interest. Very similarly, U.S. Pat. No. 5,492,126, to Hennige et al, relates to a probe comprising a combination of therapy and imaging scanning transducers. Both transducers can be single element transducers or array devices. Each transducer has a specific geometry based on the particular application and the transducers are placed one next to the other. This configuration has large dimensions and thus requires additional room to be installed.
U.S. Pat. No. 6,050,943, to Slayton et al., discloses an air-backed transducer array assembly capable of simultaneously generating high power ultrasound, forming an image of the area of interest and monitoring the temperature of the tissue being sonified. The transducer assembly is equipped with a fluid cooling system mounted on the front face of the transducer. The ultrasonic transducer assembly is said to be useful in a combined diagnostic-therapy modality. However, performing all operations with the same ultrasonic device requires that the system alternately supply the area of interest with either low intensity, wide bandwidth signals or high intensity, narrow bandwidth signals and it would appear to be difficult to reconcile the two modes of operation. Furthermore, the patent does not address a potential compatibility problem with respect to piezoelectric material used. Commonly, as indicated above, ceramics suitable for high power applications are not suitable for imaging operations and vice versa. Finally, if a compromise is made with respect to the material used, this can lead to excessive heating of the device when used in a High Intensity Focused Ultrasonic (HIFU) mode, whereas when used in the imaging mode, the quality of image produced will be weaker than that provided by standard transducer devices available in the marketplace.
To overcome drawbacks set forth above, there is provided, in accordance with the present invention, a multipurpose ultrasonic transducer for general use both in high resolution imaging and in therapy or other high intensity applications. In general, the ultrasonic transducer apparatus is constructed as a sandwich of two transducer arrays, each having a respective resonance frequency so the transducer apparatus can be operated and controlled as a single layer transducer or as a multilayer transducer. The ultrasonic probe formed by the transducer apparatus is of a construction that enables the probe to be of compact size and to offer superior electro-acoustic performance in comparison with conventional prior art ultrasonic probes.
In accordance with a first aspect of the invention, an ultrasonic probe is provided for use in combined imaging/therapy or in a HIFU mode or system. The transducer is comprised of a first imaging transducer array operating at a first frequency and a second transducer array operating at a second frequency, the first and second transducer arrays being integrated in an inter-digital manner and having approximately the same acoustic aperture, so that the system control software is simplified.
In accordance with a second aspect of the invention, an ultrasonic probe is provided which is comprised of a transducer unit comprising a first transducer array having a first thickness and a second transducer array attached to a rear or back surface of the first transducer and having a second thickness, the transducer unit having a thickness corresponding to the sum of the thickness of the first transducer array and the thickness of the second transducer array. The polarities of the first and second transducer arrays can be reversed in order to enhance the electrical impedance of device.
In accordance with a third aspect of the invention, the first transducer array and the second transducer array are produced from a common original piezoelectric member, and each transducer array is formed to have a thickness corresponding to its array frequency.
Finally, in all of the above aspects of the invention, the first and second transducer arrays are independently connected to the respective system cables therefor and can thus be separately addressed by the system.
Among other advantages, an ultrasonic probe as set forth above is capable of simultaneously performing high resolution imaging processes as well as steering and focusing high intensity ultrasonic energy in the area of interest. The interdigitated integration of the two arrays provides a powerful approach to driving the transducer electronically, in that the same delay lines can be used for one array and the other array without any inconvenience. According to the transducer construction, separating two adjacent high intensity transducer elements with an imaging transducer element therebetween significantly increases the thermal dissipation of the particular array being excited, and further, cross-coupling between elements is also dramatically reduced, thereby improving the overall quality of system.
Ultrasonic devices in accordance with the invention can be specifically constructed for performing diagnoses of images combined with either drug delivery, or harmonic imaging, or HIFU operation.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.